The present invention is a read system for reading information from a storage medium and for providing an output signal to circuitry external from the read system. More particularly, the present invention is a voltage bias, current sense preamplifier for use with a magnetoresistive reader.
There are presently two types of disc drive systems which write information to and read information from a magnetic storage medium, such as a disc. First, there is an inductive write, inductive read system. Second, there is an inductive write, magnetoresistive (MR) read system. It is the second category in which the present invention lies.
The front end of a disc drive system typically consists of one or more read/write transducers (recording heads), an electronics module (containing the read preamplifier and the write driver), and interconnections between the various heads and the module. The module is placed close to the head to keep the interconnections as short as possible.
The present trend in the data storage industry is to increase aerial density on a magnetic storage medium at constant or even decreasing latencies. This results in magnetic storage mediums having narrower tracks, larger linear densities and higher data rates. While a single-element inductive read/write head has the attraction of simplicity, its applications are becoming outdated due to a non-adequate bandwidth. The bandwidth of a head directly affects the speed with which a head can read information from a magnetic storage medium. The larger the upper pole of the bandwidth, i.e., the point at which the gain of the head begins to roll off, the faster the head can read information from the magnetic storage medium.
In traditional inductive read/write heads, there is a severe conflict in choosing the ideal number of coil "turns" for read and write operations. Narrower track widths require a larger number of turns for reading. This makes the coil inductance increase quadratically. The resonance frequency of the coil inductance and the coil/wiring/electronics capacitance therefore decreases linearly. This reduces the useful data bandwidth rather than increasing it to accommodate a higher data rate. The use of a MR read element does not present this bandwidth restriction. It also allows separate optimization of the MR read element and the inductive write element, making possible write-wide, read-narrow strategies.
A preamplifier that senses a signal out from a MR read element is fundamentally different than a preamplifier which senses a signal from an inductive read sensor. The inductive read sensor has no DC bias across it so that a preamplifier can be directly coupled to it to sense the signal from DC frequencies up to the required upper bandwidth. An MR preamplifier, however, must have the ability to compensate for an inherent DC offset across the sensor which is required to properly bias the MR read element, thereby producing a linear output signal. This bias is on the order of a few hundred milli-volts so that a high gain amplifier that amplifies DC signals cannot be directly connected to the sensor. If such a connection were made, the preamplifier would sense this offset and saturate the amplifier. Therefore, a preamplifier which is connected to an MR read element should pass an AC signal representing information from the magnetic storage medium, but not past the DC biasing signal used to bias the MR read element.
In conjunction with a single strip MR read element, the MR read element, a preamplifier, and a bias current generator are formed in a series arrangement between two supply terminals. Thus, the current supplied by the bias current generator is fed to the MR element so as to bias the MR element. The current supplied to the bias current generator is also fed to the preamplifier circuit. This bias current through the preamplifier circuit results in a certain noise contribution. Prior art preamplifier circuits utilize a capacitor connected between a low voltage potential terminal and a gate of a MOSFET or jFET transistor within the individual channel circuit. Thus, the unwanted DC signal is eliminated during a read operation. However, the MOSFET or jFET transistor of the preamplifier circuit provides a substantially high level of unwanted noise into the system and prevents accurate reading of the desired signal.
In conjunction with a dual strip magnetoresistive head, which includes two separate magnetoresistive elements, prior art preamplifier circuits utilize a cross-coupling design which cross-couples two capacitors between two separate MOSFET or jFET transistors within the individual channel circuit. Each MOSFET or jFET transistor corresponds to one of the MR elements similar to the single strip MR design. Once again, the two MOSFET or jFET transistors provide a substantially high level of unwanted noise to the system which prevents accurate reading of the desired signal. In addition, multiple capacitors per individual channels were necessary.
Therefore, there is a need for a preamplifier circuit which will block any unwanted DC signals during a read operation and will minimize the amount of unwanted noise within the preamplifier circuitry using a minimal amount of components.